The present invention relates to a method and to an apparatus for the monitoring of electric lines for short circuits. In particular the invention relates to a method and an apparatus of this kind such as are used in the monitoring of so-called OSSD lines (output-safety-switching device lines) of protective devices for the monitoring of protected regions.
In order to secure dangerous locations, such as are, for example, formed by fully automatic machines, presses and the like, use is made of preferably protective devices having object detection units, in particular object detection units which are operating in contact-free manner, for example in the form of light grids.
In this respect it must be ensured that, in the event of an object entering into the protected region, a corresponding switching signal is sent to the machine control so that the machine operating in the protected region is automatically switched off.
Depending on the danger potential of the machine, the entire system, i.e. the protective device with the object detection unit, machine control and also the connection between these two units, must meet a specific safety classification in order to still be able to switch off reliably when possible faults occur in part of this system. In this respect, for systems of the safety class 4, the requirement exists, amongst other things, that the electric lines, via which the switching signal is transmitted from the protective device to the machine control, must be present in duplicate in redundant manner, so that a fault on one of these lines is reliably recognized by a comparison with the fault free line.
In this respect it is problematic that cross connections, i.e. short circuits between the two lines, cannot be recognized by a comparison of the switching signals applied to the lines, because these are in all cases identical in the event of a cross-connection. In order to be able to ensure also the required safety for this part of the monitoring device, previously complicated measures, such as, for example, a dual laying of the lines, the use of screened lines or a laying of lines in a conduit, were necessary.